fix: replace all remaining old brand tokens (text-brand-dark, border-brand-border, bg-white opacity)

30 references to removed CSS variables: border-brand-border → border-[#1e2130],
text-brand-text-muted → text-[#4f5666], text-brand-dark → text-white,
bg-white/[0.04] → bg-[#191c25], hover:border-white/[0.12] → hover:border-[#2a2f3d].

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/assistant/ChatMessage.tsx

@@ -29,7 +29,7 @@ export function ChatMessage({ role, content, suggestedFlows }: ChatMessageProps)
           className={`rounded-2xl px-4 py-3 text-[0.875rem] leading-relaxed ${
             role === 'user'
               ? 'bg-primary/15 text-[#e2e5eb]'
-              : 'bg-white/[0.04] text-[#e2e5eb] border border-brand-border'
+              : 'bg-[#191c25] text-[#e2e5eb] border border-[#1e2130]'
           }`}
         >
           <MarkdownContent content={content} className="text-[0.875rem] leading-relaxed" />